1. Field of the Invention
The present invention relates to a real-image finder optical system.
2. Description of the Related Art
A real-image finder optical system, provided independently from a photographing optical system in, for example, a compact camera, typically includes an objective optical system having positive power, an image-erecting optical system which erects an inverted image formed through the objective optical system to attain an erect image, and an eyepiece optical system for magnifying and observing the erect image, in this order from the object. Furthermore, in many cameras which include a real-image finder, a display member (field frame member) is provided in the vicinity of the image forming plane of the objective optical system in order to display pattern information, such as a field frame and the like within the finder. In general, a display member is formed to indicate predetermined information, such as a field frame,an object distance measuring zone, photographing modes, other numerical information and the like, in the finder view-field by blocking incident light from the object via non-transparent patterns corresponding to predetermined information formed on a transparent member. An liquid crystal display (LCD) is utilized in order to display variable data.
However, in a real-image finder of the prior art, the display member is located in the vicinity of the image forming plane of the objective optical system, on which a viewer's eye is focused. Therefore, if foreign matter such as dust, sticks to the display member during assembly of the finder, such foreign matter can be easily viewed through the finder. In order to lessen the probability of foreign matter being apparent when viewing through the finder, optical elements are removed from the vicinity of the image forming plane, or the number thereof are reduced. Accordingly, in the prior art, attempts have been made to provide optical elements which constitute an objective optical system or an image-erecting optical system at a distant location from the image forming plane. However, since the finder of the prior art is constructed so that the display member is directly viewed through the eyepiece optical system, in order to clearly observe the pattern information of the display member, the display member has to be close to the image forming plane. Furthermore, the problem of foreign matter obstructing the finder view could not be satisfactorily solved.
Japanese Unexamined Patent Publication No. Hei-9-113962 has disclosed a real-image finder which can display various items of pattern information as a virtual image by utilizing an optical system including a half mirror surface. With such a finder, foreign matter which may have stuck to the display member becomes no longer visible. However, as the eyepiece optical system and the display member are distant from each other, when a viewer observes the finder field-of-view by placing his/her eye distant from the eyepiece optical system, any foreign matter stuck to the display member becomes apparent. Also, due to the focal length of the eyepiece optical system being increased, magnification of the finder is decreased, thereby causing a further problem of the object being more difficult to observe.
It is well known in the art that a half mirror surface for obtaining a virtual image is formed as an emitting surface (a rear reflection surface) of a prism (glass or plastic) where light exits from the inside of the prism to the surrounding air. It is also known that the rear reflection surface is more influenced by the surface precision of the half mirror surface than a typical incident surface (a front reflection surface) where light is incident from the surrounding air to the inside of the prism. The half mirror surface in an embodiment taught in Japanese Unexamined Publication No. Hei-9-113962 is not only formed as a rear reflection surface of a prism, but also is curved.
In such a prism which includes a curved surface, it is conventional to produce the same via injection molding using a plastic material. However, it is extremely difficult to produce a product having a reflection surface with such high precision via an injection molding process. Furthermore, even if a successful molding is made, the plastic material used therefor has a tendency to change a refractive index in the prism, and at the same time, change the shapes of reflection surfaces and light transmitting surfaces according to changes in temperature and humidity. Consequently, it is difficult to maintain the precision of the prism.
According to Japanese Unexamined Publication No. Hei-9-113962, any foreign matter stuck to the display member is not directly visible through the finder. However, since the prism disclosed in this publication contains a half mirror surface in an optical path, the ghost light, scattered from the reflection surfaces thereof and visible in the field-of-view as ghost images, is much more frequently observed than in a real-image finder which does not usually have a half mirror surface. In other words, because of the existence of the half mirror surface, although JUP No. Hei-9-113962 has disclosed the real-image type finder, the ghost light is a problem as in a virtual-image type finder, in particular an Albada type finder, which usually has a half mirror surface in an optical path. It should be understood that if an arrangement of optical elements is similar to that of Japanese Unexamined Publication No. Hei-9-113962, ghost images observed in a virtual-image finder will become a problem.